Build a retaining wall that holds up

Digging is tough part of project

August 12, 2005|By Mike McClintock, Special to the Tribune

A good retaining wall is an unlikely combination of designs--basically a strong wall full of holes. It has to be strong enough to hold back earth that can exert tremendous pushing force. It also has to be porous enough to let water pass through.

Unless you get both parts right, retaining walls fail. Some just fall over. Worse yet, heavy masonry walls often crack and tip into positions that look wildly unstable yet somehow hang there--impossible to push back into place and extremely difficult to dismantle.

Construction choices

You can avoid these problems by constructing a balanced system using several different materials and configurations. Common choices include wood (pressure treated and on the scale of railroad ties), poured concrete, concrete block and proprietary precast blocks sold in different shapes, textures and colors.

Then there's the question of who builds the wall. It's a great, if rugged, do-it-yourself project in good weather. But there's a major catch--the digging, which often looms much larger than building the wall itself. Heavy masonry walls require a footing, which means excavating into undisturbed soil that's hard to dislodge. Retaining walls also last longer when there is a gravel-filled drainage trench on the high side, which means even more digging.

Look at the project realistically--one DIY shovelful at a time--and the idea of hiring a pro may start to look more and more sensible. Contractors may not build a better wall. But they typically bring in small-scale trenching machines and backhoes that make quick work of all the earth moving.

Here is a look at the different systems so you can zero in on the best solution for your sloping site.

Landscape timber walls. A wall of stacked timbers, generally sloping for some gravitational advantage, is anchored into the bank by inserting several perpendicular timbers, called deadmen.

The timbers are tied together with spikes and rely on the deadmen to resist tipping. (Packing them into the bank at right angles to the wall can be backbreaking work.) Water tends to flow freely though the many joints, though sometimes a fair amount of silt passes through as well. But there's no need to build a drainage trench behind the wall or to install weep holes in the face of the wall.

Poured concrete and concrete block. Block-and-mortar construction is straightforward but, liked a poured wall, requires a concrete footing. To provide the greatest resistance to tipover, voids in blocks should be filled with concrete and further strengthened with reinforcing rods. Building the forms for a poured concrete wall is even more complicated.

Both these solid masonry systems are strong enough to hold up a house--and kind of a major production just to hold back some dirt. Both also require two drainage assists designed to reduce hydrostatic pressure, the main reason retaining walls topple.

The first is a drainage trench. It might be just a foot or so of gravel, which draws off more water than dirt. Longer-lasting trenches also include a drainpipe to carry away water and plastic sheeting or filter fabric to keep dirt from clogging the gravel.

The second is a system of water-releasing weep holes every 3 or 4 feet along the base of the wall. Some installers use small-diameter plastic tubes to minimize the visual impact on a brick or stone facade. But larger pipes (often 1 1/2-inch-diameter plastic) extending through the wall into the gravel release more water. They are also less likely to clog, particularly when the gravel end is protected with screening.

Precast block. These prefinished (textured) proprietary systems stack and interlock securely without mortar. A typical system has predrilled holes where you insert short plastic pegs between blocks. Many have more than one set of connections, which allows you to increase or decrease the slope of the wall.

You also can assemble most precast blocks into a gently curved wall, which is much more difficult to do with poured concrete or concrete block. To create staggered joints you'll need half blocks or corner blocks. In systems without these extras full blocks must be cut with a masonry saw. But overall assembly is straightforward--you just keep stacking.

With mortarless joints, water drains through the wall so there is no need for weep holes. But to minimize the seeping of silt, backfill with gravel before replacing the dirt.

Overall, these popular systems provide that unlikely combination--a strong but porous wall--that's easy to build and likely to stay where you put it.